The invention relates generally to multi-wavelength optical switches. In particular, the invention relates to optical switches using micro electromechanical system (MEMS) switching elements.
Modern communications networks, particularly those extending over long distances, increasingly use silica optical fiber as the transmission medium. In the originally implemented fiber-based networks, each fiber carries a single optical carrier at a wavelength in one of the silica transmission bands that extend across ranges in the neighborhoods of 850, 1310, and 1550 nm. At the transmitting end, a laser emitting at this wavelength or an associated electro optical modulator is modulated by an electrical data signal, and the modulated narrow-band light is input to the fiber. At the receiving end of the fiber, a photodetector receives the modulated light and converts it back to electrical form. While the fiber itself has a transmission bandwidth measured in hundreds of terahertz, the data transmission rates are limited to the speed of the electronics associated with the transmitter and receiver, currently about 10 gigabits per second. It was quickly recognized however that the transmission bandwidth of a fiber can be greatly increased by wavelength division multiplexing (WDM). For example, W lasers at the transmitting end, where W may be forty or more, output at respective ones of W wavelengths in one of the silica transmission bands, and their outputs are modulated by respective data signals. The wavelength spacings in the 1550 nm band may be 1 nm or less. All the modulated optical carriers are combined and input to a single transmission fiber. At the receiving end of the fiber, a wavelength dispersive element such as a diffraction grating or prism wavelength divides the received multi-wavelength optical signal into W respective spatial paths. A photodetector and associated electronics are associated with each of these paths. Thereby, the transmission capacity of the fiber is increased by a factor of W because of the parallel operation of W sets of electronics.
Modern communication networks tend to be more complicated than the point-to-point system described above. Instead, most public networks include multiple nodes at which signals received on one incoming link can be selectively switched to different ones of outgoing links. For electronic links, conventional electronic switches directly switch the electronic transmission signals. Fiber links present a more difficult switching problem.
In the most straightforward approach, each node interconnecting multiple fiber links includes an optical receiver which converts the signals from optical to electrical form, a conventional electronic switch which switches the electrical data signals, and an optical transmitter which converts the switched signals from electrical back to optical form. In a WDM system, this optical/electrical/optical (O/E/O) conversion must be performed by separate receivers and transmitters for each of the W wavelengths. This replication of O/E/O components prevents the economical implementation of WDM for a large number W of wavelength channels.
Another approach implements wavelength switching in an all-optical network. In a version of this approach that may be used with the invention, the W wavelength components from an incoming multi-wavelength fiber are wavelength demultiplexed into the different spatial paths. Optical switching elements then switch the wavelength-separated signals in the desired directions before a multiplexer recombines the optical signals of differing wavelengths onto a single outgoing fiber. In conventional terminology, all the optical switching elements and associated multiplexers and demultiplexers are incorporated into a wavelength cross connect (WXC), which is a special case of an enhanced optical cross connect (OXC). Advantageously, all the optical switching elements can be implemented in a single chip of a micro electromechanical system (MEMS). The MEMS chip includes a two-dimensional array of tiltable mirrors which may be separately controlled. Solgaard et al. describe the functional configuration of such a MEMS wavelength cross connect in U.S. Pat. No. 6,097,859, incorporated herein by reference in its entirety. Each MEMS mirror receives a unique optical signal of a single wavelength from an incoming fiber and can switch it to any of multiple outgoing fibers. The entire switching array of several hundred mirrors can be fabricated on a chip having dimension of less than 1 cm by techniques well developed in the semiconductor integrated circuit industry.
However, such a wavelength optical cross connect needs to be installed in the field and to retain its calibration under somewhat harsh conditions without the need for frequent routine maintenance. Its packaging should be relatively compact to allow its installation in existing switching facilities and in perhaps remote locations. The cost and complexity need to be minimized.
Smith et al. in U.S. patent application Ser. No. 09/957,312, filed Sep. 20, 2001, incorporated herein by reference in its entirety, sketchily discloses a more compact package including condensed physical optics such as folding mirrors. A similar disclosure is published as International Publication No. WO 02/25358 A2. However, that description is directed more to features other than the optics.
A wavelength cross connect advantageously is connected to many optical fiber transmission links, and the number of WDM wavelengths is also advantageously large. The design of the wavelength switching system becomes increasingly difficult for a large number of input/output fibers and a large number of wavelengths. Further, for a large number of fibers and wavelengths, it becomes increasingly difficult to fabricate all the required MEMS mirrors in a single substrate.
A optical cross connect (OXC) is based on free-space optics and an array of micro electromechanical system (MEMS) mirrors for selectively switching optical signals between waveguides, such as optical fibers. For a wavelength optical cross connect (WXC), the MEMS mirrors may be arranged in a two-dimensional array, preferably within the same plane and more preferably within a same substrate. The two-dimensional mirror array extends in a fiber direction and in a perpendicular wavelength direction.
Transmission fibers are coupled into the free-space optics through a concentrator which couples on a first side to the fibers spaced by distances representative of the diameter of single-mode optical fiber, for example, at least 125 xcexcm. The concentrator includes optical waveguides which curve so that the distances between the waveguide decrease from the first side of the concentrator to the second side which has an output facet to the free-space optics. At the second side, the waveguides are arranged in a linear array spaced by a much smaller distance, for example, 20 to 50 xcexcm and couple to beams arranged around respective parallel axes in a plane. The concentrator waveguides may be planar waveguides formed in a substrate, or they may be optical fiber aligned to curved grooves formed in a substrate and preferably having ends tapered at the input to the free-space optics.
The free-space optics may be arranged in and about a principal optical plane. The fibers or other waveguides are preferably arranged in one or more linear arrays extending a small distance perpendicular to the principal plane, for example, as determined by the concentrator, and inputting parallel beams to the free-space optics. More preferably, both input and output waveguides are arranged in a single linear array. More preferably also, a folding mirror reflectively couples selected pairs of mirrors in the MEMS array with one mirror acting as an input mirror and the other as an output mirror. The tilts of the selected input and output mirrors are controlled in pairs to produce the reflective coupling. Further, the MEMS array may be inclined, for example, at 45xc2x0 to the principal plane to allow the folding mirrors to be placed parallel to the principal plane.
The free-space optics may include a collimating lens system, preferably including a field-flattening element. A wavelength dispersive element, such as a diffraction grating, may be placed between the collimating lens system and the MEMS array to separate wavelength components of the waveguided signals. A focusing lens is preferably disposed between the wavelength dispersive element and the MEMS array and has a focus point near the MEMS array. If a fold mirror is used, the focus point is on or near the fold mirror. Alternatively, the focus point may be on the MEMS array or between it and the fold mirror. Closely spaced waveguides ends are imaged onto the MEMS array with a magnification determined by the ratio of the focal lengths of the collimating lens system and the focusing lens. The magnification is preferably between 10 and 100, and more preferably between 20 and 50. The optics preferably are designed to be telecentric producing parallel beams and sub-beams, at least in the wavelength direction.
The free-space optics may include a prism to compensate for the astigmatism of the wavelength dispersive element, such as a diffraction grating. By the use of multiple mirrors, the free-space optics may be arranged along a twisted optical axis that crosses itself, thereby reducing the size of the package.
The scaling of the invention to larger number of fibers or of wavelengths can be eased by use of multiple MEMS arrays. Multiple MEMS arrays can be bonded onto a common substrate, thus forming a mosaic array. Separate MEMS arrays can be used for input and output mirrors, and the input and output mirrors of the two arrays may be directly coupled without the need for a fold mirror. Separate sets of optics and waveguide arrays may be used on the input and output with the two arrays. Alternatively, the wavelength separated sub-beams produced by the wavelength dispersive element may be grouped and directed through different sets of back-end optics and MEMS arrays.
The scaling can also be facilitated by splitting the waveguide signals into two or more parallel paths by means of one or more stages of divider/combiners, such as power splitters or wavelength interleavers. Free-space optical or wavelength interconnects optically switch different wavelengths while assuring that the unswitched wavelengths are absorbed or otherwise do not interfere.
The invention also includes the method of operating such optical cross connects.